This invention relates to a method for preparing a cross-linked polymeric composition, and more particularly, it relates to a method for preparing a cross-linked polyimide polymer and to articles formed from the cross-linked polymer.
Polyimide resins, especially polyetherimide resins, are well-known for their use in wire-coating applications. These resins generally provide good solvent resistance and thermal stability when applied to electrical conductors such as magnet wire. One highly regarded polyetherimide resin film used for this type of application is the ULTEM.RTM. product of General Electric Company. Although the ULTEM.RTM. product can be easily applied to electrical conductors, the increasing use of high-speed extruding and coating systems requires that polymeric compositions cure more quickly and possess greater thermal stability than the prior art compositions. Greater thermal stability is desirable in wire insulation used in certain electrical devices, such as motors and transformers, because these devices frequently function at temperatures greater than the creep point of prior art thermoplastic polyetherimide coatings, thereby causing such coatings to begin to deteriorate, possibly resulting in the failure of the electrical devices.
The problem of high-temperature stability in polyetherimide compositions is addressed in U.S. Pat. No. 3,983,093. U.S. Pat. No. 3,983,093 relates to the copolymerization of more rigid dianhydrides such as pyromellitic dianhydride with 2,2-bis[4-(2,3-dicarboxyphenoxy)phenyl]propane dianhydride (bisphenol A dianhydride or BPADA). Although the use of the more rigid dianhydride enhances the high temperature capabilities of the polymer, the solubility of the polymer in nonpolar solvents may be decreased. Thus, dipolar solvents may be necessary, but these solvents are more difficult to handle because of their increased toxicity.
In U.S. Pat. No. 4,157,996, methylene dianiline was used with BPADA to form a polyetherimide-type coating solution useful as an insulator on copper and aluminum wire. Although these materials have good thermal stability and solvent resistance, it is desirable to provide a process which further improves the thermal stability and solvent resistance of the polyetherimide material.
There is disclosed in U.S. Pat. No. 4,115,341 a process for preparing polyetheramide-acidimide polymers which are based on BPADA and methylene dianiline. Although these polymers exhibit good thermal stability and are suitable for solvent deposition wirecoating methods, it is desirable to provide coatings which exhibit even greater thermal stability and which may be solvent-deposited or extruded in the solventless melt phase onto electrical conductors.
It is therefore an object of this invention to provide a process which improves the solvent resistance of polyimide-type compositions used in wire-coating applications.
It is a further object of this invention to provide an improved process for increasing the thermal stability of polyimide-type compositions without sacrificing the solubility of such compositions in nonpolar solvents.
It is still a further object of this invention to provide a polyimide composition which can be used to coat electrical conductors by either solvent deposition or solventless extrusion.
It is yet another object of this invention to provide a metal conductor having thereon a cross-linked polymer which exhibits improved thermal stability and solvent resistance.
Other objects and advantages of the invention will become apparent as the description thereof proceeds.